Lake Okeechobee water level falling due to evaporation

OKEECHOBEE — “Why are they letting all the water out of the lake?”

A common complaint this time of year comes from newcomers and visitors who don’t seem to understand that it’s Mother Nature — not water managers — lowering the lake level, by sending water into the atmosphere through evaporation and evapotranspiration and into the Earth to recharge the aquifer through percolation.

Fishermen who complain that the lake is low also don’t seem to understand that the natural ecosystem of Lake Okeechobee needs periods of lower lake levels, which allow the vegetation around the lake’s littoral zone to regrow. That vegetation provides critical spawning areas for fish, and cover for young fish. Anglers who understand the lake’s ecosystem welcome droughts as part of the natural process that keeps the fisheries healthy.

While much of the water that enters the lake is controlled by the South Florida Water Management District’s locks, culverts and other water control structures, there are no human controls on most of the water that leaves the lake.

“Ideally, Lake Okeechobee should never rise above 15.5 feet and should drop to about 12 feet most dry seasons,” states “Lake Okeechobee: A Synthesis of Information and Recommendations for its Restoration,” by P.N. Gray, C.J. Farrell, M.L. Kraus and A.H. Gromnicki. The study, published in 2005 by Audubon of Florida, was funded in part by the Batchelor Foundation and by the Everglades Foundation.

What’s the difference between evaporation and evapotranspiration? Webster’s Dictionary explains that evaporation is the movement of water to the air from sources such as the soil, canopy interception and water bodies. Evapotranspiration is the sum of evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere.

Evaporation and evapotranspiration rates are difficult to measure. Scientists use different models. On average, they estimate Lake Okeechobee drops by between 4.3 feet and more than 5 feet each year due to the movement of water into the air.

A study conducted in Florida in 1995 by the United States Geological Survey found there are many factors in play, including the temperature of the air, temperature of the water, depth of the water, temperature of the soil under the water, and horizontal and vertical wind speeds. Water evaporates more quickly in open water, less quickly in marshy areas.

Soil composition and types of vegetation affect evapotranspiration rates. Evaporation rates in open water increase on a sunny, cloudless day.

“Evaporation Estimation for Lake Okeechobee in South Florida,” by Wossenu Abtew, published in the June 2001 edition of “Journal of Irrigation and Drainage Engineering,” analyzed meteorology data from 1993-1997 and estimated a loss of 132 centimeters per year (about 4.3 feet) from evaporation.

“Evapotranspiration Estimate for South Florida” published in 2003 by Wossenu Abtew, Jayantha Obeysekera, Michelle Irizzary-Ortiz, Danielle Lyons and Anna Reardon, found that average evaporation rates in the South Florida Water Management District range from 122 cm (about 4 feet) in the north part of the SFWMD to 137 cm (about 4.5 feet) in the south.

“Evaporation and evapotranspiration are functions of solar radiation, temperature, wind speed, vapor pressure deficit, atmospheric pressure, characteristics of the surrounding environment and type and condition of vegetation. South Florida is an area of high rainfall, high humidity and generally low wind speed. Air temperature is high with relatively warm winter months. Solar radiation is abundant with seasonal and daily variation,” the authors state. Annual lake evaporation in the continental United States ranges from 51 cm in the extreme northeast to 218 cm in Southern California, the authors note.

The 2005 Audubon study estimated water loss to evaporation at around 5 feet per year — usually higher during dry periods.

“In an average year, Lake Okeechobee receives enough rain and inflow to raise it about 7.5 feet (note: this discussion of the water budget uses rounded numbers for simplicity).

This water does not come in at once, but is spread over the entire year, thus the lake does not actually rise 7.5 feet each year. Roughly 5 feet of this water will evaporate, leaving 2.5 feet of water in the lake. On average, irrigation and municipal demands use 1 to 2 feet of lake water per year, leaving an “excess” of about one foot of water that must be discharged,” the 2005 Audubon report explains.

During the dry season, when rainfall does not replace the water lost to evaporation, the lake level falls. During the wet season, when water comes in faster than it can go out, the lake level rises.

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